In construction applications, mortars may be prepared with cement, sand, and organic polymer. To reduce shipping costs, the polymer can be shipped and added in dry form as a redispersible polymer powder. Redispersible polymer powders are used as binders to improve the adhesion of cementicious adhesive formulations. The powdered form of the polymer is generally produced by spray drying a liquid polymer composition to obtain a free flowing powder. To perform its function in the application formulation to which it is added, such as concrete, it is desired that in the application formulation the polymer powder is easily redispersible. Redispersible styrene butadiene (SB) latex based powders provide some advantages in cementicious applications such as saponification resistance and cohesive strength when compared to redispersible vinylacetate (VAc) and vinylacetate ethylene (VAE) based powders. The typical polymer particle sizes of the SB latexes and the particle size of redispersed SB powders is significantly smaller compared to VAc/VAE latexes and their redispersed powders. Although a small particle size can be an advantage for the application, it also impacts the latex particle stability and the redispersed powder stability. The stability is a very critical factor in cement based applications due to the high pH and the high multivalent ion load. At the same latex level the available surface area is increased by a factor two when decreasing the particle size by 50%. In a non-critical environment, the stability of SB-latex, achieved by the carboxylation and surfactant, is sufficient. Liquid SB-latex used in cementicious applications needs to be additionally sterically stabilized with large amounts of surfactants. Unfortunately this approach leads to a significantly reduced redispersibility after spray drying due to the high surfactant amount.
The stabilization of redispersed polymer powder is also improved by the polyvinyl alcohol which is used as a spray drying aid for the production of the redispersible powder. However, the stabilization improvement of currently employed amounts of polyvinyl alcohol is not sufficient in the case of SB latexes even though the same amount of polyvinyl alcohol imparts stability to VAc/VAE latexes. Larger levels of conventionally employed PVOH, such as conventional partially hydrolyzed polyvinyl alcohol (PVOH) might improve the stability, but the required additional quantity would be beyond economically and technically justifiable limits. For example, for effective redispersibility a large amount of PVOH may be needed but it tends to adversely increase the viscosity of the polymer composition or dispersion making it difficult to produce a powder by spray drying. Moreover, at high pH values that are generally employed in polymer compositions or dispersions for making redispersible powders for high pH application formulations, such as cement formulations, partially hydrolyzed PVOH may continue to hydrolyze, reducing effectiveness of PVOH as a protective colloid. In all cases the stabilization of polymer particles is an indispensable prerequisite for excellent performance in cement based applications.
International Patent Application Publication WO 2008/133375 to Jeong Woo Son discloses that generally, when emulsion polymerization is performed using monomers such as acrylic monomers and styrenic monomers, an anionic or nonionic surfactant is used as an emulsifier. However, the emulsion prepared by using a surfactant has poor chemical stability when combined with an electrolyte, and as a result, if it is added to cement, or the like, fluidity of the cement composition is deteriorated. In order to overcome such problem associated with the use of the surfactant, it is disclosed, polyvinyl alcohol could be used as a protective colloid. In this case, the chemical stability was improved, but the polymerization stability was deteriorated, thereby still entailing a problem that a stable emulsion is hardly obtained. As disclosed in WO 2008/133375 there has, accordingly, been proposed a method involving using modified polyvinyl alcohol in which a functional group such as a mercapto group is introduced to one end, as a protective colloid, or a chain transfer agent in combination, to improve the polymerization stability. However, it is disclosed, this method leads to a reduced degree of polymerization, and as a result, it cannot satisfy water resistance, mechanical strength, durability, or the like. The WO 2008/133375 invention provides an acrylic acid ester copolymer emulsion composition, and redispersible powders made therefrom. According to WO 2008/133375, use of a polyvinyl alcohol having a degree of saponification of 85 mol % or more, and an average degree of polymerization of 300 to 1400; hydrophilic ethylenic unsaturated monomers having a water solubility of 1% or more; hydrophobic ethylenic unsaturated monomers having a water solubility of less than 1%; and a lipophilic initiator provides an acrylic acid ester copolymer composition which has excellent polymerization stability, and improved water resistance, alkali resistance, and fluidity. It is further disclosed that the redispersible powders prepared by spray drying of the acrylic acid ester copolymer composition have improved water redispersibility, and thus, can be used in various fields such as an additive to a hydraulic material, a powder paint, and an adhesive. However, production of a styrene butadiene based redispersible polymer powder which is both readily redispersible and stable in cement applications is not disclosed.
In the present invention, it has been surprisingly found, that use of a modified polyvinyl alcohol having an alkyl end group, as a colloidal stabilizer or spray drying aid for styrene butadiene latexes unexpectedly provides both excellent redispersibility and significant stability improvements in cement applications.